1. Field of the Invention
The present invention relates to a continuously variable transmission system for a vehicle employing a combination of a continuously variable transmission and a planetary gear train and, in particular, to a clutch control system therein.
2. Description of the Related Art
Such a continuously variable transmission system for a vehicle is known in Japanese Patent Application Laid-open No. 10-196759.
In accordance with this continuously variable transmission system for a vehicle, when the vehicle is traveling at a low speed the engine torque is transmitted to the driven wheels via a toroidal type continuously variable transmission alone, and when the vehicle is traveling at a high speed the engine torque is transmitted to the driven wheels via both the toroidal type continuously variable transmission and a planetary gear train, and a wide range of ratios that cannot be achieved by use of the toroidal type continuously variable transmission alone can thereby be achieved.
Japanese Patent Application Laid-open No. 9-210168 discloses a toroidal type continuously variable transmission in which, by controlling the engagement force of a clutch provided on the output shaft according to the shift ratio, the energy loss accompanying operation of the clutch is reduced, thereby maintaining high efficiency. Japanese Patent Application Laid-open No. 11-257445 discloses an arrangement in which, when the control system of a belt type continuously variable transmission fails, the starting clutch is controlled by changing a control hydraulic pressure according to the rotational rate of the engine, thereby enabling the vehicle to start.
In order to control the ratio of a toroidal type continuously variable transmission, such as that disclosed in the above-mentioned Japanese Patent Application Laid-open No. 10-196759, an electronically controlled device such as a solenoid valve is used, but if the electronic control system fails, the ratio of the toroidal type continuously variable transmission cannot be controlled in some cases. If such a failure occurs when the vehicle is stationary, then when the vehicle is starting, a load that changes the ratio of the toroidal type continuously variable transmission beyond the LOW ratio or a load that changes the ratio thereof beyond the OD ratio is generated, and a large load is thus applied to the input discs or output discs of the toroidal type continuously variable transmission, thereby causing a possibility that the durability could be degraded. Furthermore, if the ratio of the transmission is fixed at the OD ratio, a torque that is transmitted to the driven wheels decreases, thereby causing a possibility that the starting performance could be degraded to a great extent or the vehicle speed could become excessive due to a high ratio when the vehicle reverses.
The present invention has been carried out in view of the above-mentioned circumstances, and it is an object of the present invention to prevent degradation in the starting performance and durability of a continuously variable transmission for a vehicle by appropriately controlling the ratio of the transmission even when the electronic control system of the continuously variable transmission fails.
In order to achieve the above-mentioned object, in accordance with an aspect of the present invention, there is proposed a clutch control system in a continuously variable transmission system, the continuously variable transmission system including a continuously variable transmission having an input member into which a torque of an engine is input and an output member to change the speed of rotation of the input member and to output the rotation; a planetary gear train having a first element, a second element and a third element, the first element being connected to the output member of the continuously variable transmission and the second element being connected to driven wheels; a first clutch to engage the first element of the planetary gear train with the second element thereof; and a second clutch for engaging the engine with the third element of the planetary gear train, wherein the clutch control system includes a first control valve that controls the degree of engagement of the first clutch in accordance with the rotational rate of the engine and the shift ratio; and a second control valve that controls the degree of engagement of the second clutch in accordance with the rotational rate of the engine.
In accordance with the above-mentioned arrangement, when the vehicle travels forward at normal conditions (i.e., when there is no failure), gradually engaging the first clutch while the engagement of the second clutch is canceled, transmits the torque of the engine to the driven wheels via the continuously variable transmission and the first clutch, thereby starting the vehicle. When the first clutch is completely engaged as the vehicle speed increases, the vehicle is accelerated as the continuously variable transmission changes from the LOW ratio to the OD ratio (direct mode). When the ratio of the continuously variable transmission reaches the OD ratio, the second clutch is engaged and the engagement of the first clutch is canceled. The torque of the engine is input into the third element of the planetary gear train via the second clutch, a majority of the torque is transmitted from the second element to the driven wheels, thereby accelerating the vehicle; and a portion of the torque is returned back to the engine side from the first element via the output member and the input member of the continuously variable transmission (torque slip mode). When the continuously variable transmission is shifted from the OD ratio toward the LOW ratio in this state, the total ratio of the continuously variable transmission system changes beyond the OD ratio of the continuously variable transmission, thereby accelerating the vehicle.
When the vehicle reverses at normal conditions (i.e., when there is no failure), gradually engaging the second clutch transmits the torque of the engine to the driven wheels via the second clutch, thereby starting the vehicle. After the vehicle has started, the second clutch is completely engaged, and the vehicle is accelerated by increasing the rotational rate of the engine.
When the vehicle travels forward at a time when the electronic control system of the continuously variable transmission has failed, the vehicle is started by gradually engaging the first clutch and the second clutch. In this case, the ratio of the continuously variable transmission is set at a predetermined ratio between the LOW ratio and the OD ratio by balancing the engagement forces of the two clutches. That is, since the first control valve controls the degree of engagement of the first clutch in accordance with the ratio of the continuously variable transmission, feedback control can be carried out so that the ratio is maintained at a predetermined ratio between the LOW ratio and OD ratio. Moreover, since the first and second control valves control the degrees of engagement of the first and second clutches respectively in accordance with the rotational rate of the engine, it is possible to ensure a required torque transmission by changing the degrees of engagement of the first and second clutches in accordance with the rotational rate of the engine. When the first clutch is completely engaged, the ratio of the continuously variable transmission changes from the predetermined ratio toward the OD ratio. Subsequently, when the second clutch is completely engaged, the rotational rate of the engine is increased while the ratio of the continuously variable transmission is fixed at the OD ratio, thereby accelerating the vehicle. During this period, the torque of the engine is transmitted to the driven wheels via the second clutch and the planetary gear train. Therefore, the continuously variable transmission only carries out the speed change and is not involved in the transmission of torque. By engaging the first and second clutches with predetermined engagement forces when the vehicle travels forward at a time when the control system has failed, it is possible to prevent the ratio of the continuously variable transmission from changing beyond the LOW ratio or the OD ratio. Accordingly, generation of an excessive load can be prevented, thereby enhancing the durability of the continuously variable transmission.
When the vehicle reverses when there is a failure, the gradual engagement of the second clutch transmits the torque of the engine to the driven wheels via the second clutch, thereby starting the vehicle. After the vehicle has started, the second clutch is completely engaged, and the rotational rate of the engine is increased, thereby accelerating the vehicle. In this case, since the torque of the engine does not pass through the continuously variable transmission, it is possible to prevent degradation of the durability of the continuously variable transmission due to an excessive load.
In the cases of starting to travel in both the forward and reverse directions, since the ratio of the continuously variable transmission is not fixed at the OD ratio, the torque to be transmitted to the driven wheels does not decrease, which would greatly degrade the starting performance, and the vehicle speed does not become too high due to a high ratio after the vehicle has started to reverse.
The first element, the second element and the third element of the present invention correspond to a sun gear 39, a ring gear 40 and a carrier 41 respectively of a first embodiment. The first element, the second element and the third element of the present invention also correspond to a carrier 41xe2x80x2, a sun gear 39xe2x80x2 and a ring gear 40xe2x80x2 respectively of a second embodiment and a third embodiment. The input member and the output member of the present invention correspond to input discs 62 and output discs 63 of the embodiments. The first control valve of the present invention corresponds to an SC back up valve 119 of the embodiments, and the second control valve of the present invention corresponds to an RSC back up valve 120 of the embodiments. Furthermore, the continuously variable transmission of the present invention corresponds to a toroidal type continuously variable transmission T of the embodiments.
The above-mentioned objects, other objects, characteristics and advantages of the present invention will become apparent from explanations of preferred embodiments that will be described in detail below by reference to the appended drawings.